The present invention relates to a power operated chuck, for turning machines, having at least one jaw which is radially movably guided in a chuck body, and which is actuated by a pressure medium piston which is movably guided in the chuck body and the working chamber of which is connected to a source of pressure medium by means of a releasable check valve.
During rotation at high cutting speeds, the centrifugal force which acts on the jaws of the chuck effects a loss of clamping force, which has a negative effect on the transferable torque and hence on the possible material removal rate. This is primarily true in the situation where work pieces are externally clamped, with jaws acting radially inwardly on the work piece from the outside. The opposite is true for internal clamping, where the jaws act radially outwardly on the work piece from within a bore of the work piece. Here the centrifugal force exerted on the jaws enhances the clamping force, which can also be undesirable because it can lead to undue deformation of the work piece and hence to undesired deviations of the shape of the surface which is being machined.
For the foregoing reasons, chucks which are to be used at high speeds are often equipped with counter weights, the centrifugal forces of which exert radially inwardly directed compensating forces on each jaw by means of suitable direction-changing means, and thus substantially or completely compensate for the change in clamping force resulting from the speed.
A further possibility for compensating for the centrifugal force is to control the hydraulic pressure in the associated clamping cylinder in conformity with the expected centrifugal force of the jaws. For this purpose, pursuant to German Pat. No. 2 150 885, an electronic control unit which is dependent upon the speed can be used; in a suitable manner, this control unit affects via a pressure regulating valve the effective clamping pressure in conformity with a predetermined operation.
A similar effect is achieved according to German Pat. No. 2 530 573 by means of pistons which are controlled by centrifugal force, are installed in hydraulic clamping cylinders of a clamping device, and increase the effective hydraulic pressure as a function of speed and without external control. In this case, exchangeable releasable check valves are used which are present in many conventional clamping cylinders as safety components.
With both of these known types of chucks, the speed dependent increase in pressure is transferred from the clamping cylinder to the chuck as an increase of the chuck operation force via a connecting member, for example a tie rod or an air tube. The additional closing force of the jaws resulting herefrom is possibly such that the speed dependent loss of clamping force of the clamping jaws is compensated for.
In addition to the widely known construction of power-operated chucks which are operated by the tension or compressive forces of a separate clamping cylinder, chucks are known which operate by applying hydraulic pressure directly or indirectly to the jaws. With such clamping tools, which are known as collect chucks, the force generating elements are accommodated directly in the chuck body. The pressure medium necessary for actuation is generally conveyed to the chuck via a rotary transmission. Here too releasable check valves are provided to protect against possible loss of the hydraulic pressure.
Basically, also with these collect chucks a compensation of the centrifugal force exerted on the jaws is conceivable by means of counter weights associated with each of the jaws, or by an externally controlled increase in pressure. With types of construction where very large bores in comparison to the outer diameter are demanded, and/or where very low overall height is demanded despite the front end or collect construction, not enough installation space is available for the required counter weights. These chucks are therefore necessarily operated at lower speeds, so that the machining capacity which is actually available cannot be utilized at high speed ranges.
Starting with a power-operated chuck for turning machines of the aforementioned general type, it is an object of the present invention to provide a simple and compact system which effectively compensates for the centrifugal force exerted on the jaws of hydraulically actuated chucks, with not only the considerable expense for an electronic control of the hydraulic pressure, but also the mechanical structural expense for counter weights and power transmission means to be avoided.